Jolenealaska is right that you are missing solids: milk powder and sugar. Other powders won't give you the traditional taste and texture, and although they may turn out to be tasty, it will need lots of experimenting.
Normal white chocolate is made of something like 1:1:1 cocoa butter to sugar to milk powder. If you change the ratio, you will have a product with a very different texture. Note that the sugar is not in there just for taste, it plays a major part in the final consistency.
You must also note that white chocolate, even when made with sufficient sugar and milk powder, can't absorb much liquid. The workable ratios are 14 to 18 ml of cream per 100 g of white chocolate. As the cocoa butter won't absorb anything, we are talking about 14 to 18 ml of cream per 66 g of combined milk and sugar; if you reduce the amount of milk powder and sugar in the chocolate, you are basically adding cocoa butter to white chocolate, making a white coverture, which has very different properties from chocolate (more waxy). If you manage to make a good tasting ganache this way, the cream will have to be measured against the milk+sugar combination, not against the whole mass.
Lecithine is an emulsifier, not a thickener. It will help you get a homogenous mass, but it will be too liquid if you don't add milk powder and sugar. Using polysaccharide thickeners with white chocolate is a bad idea, because the taste is rather subtle and it gets completely suppressed by xanthan & co. I am speaking of bitter experience here :(
Using starch might work, I haven't tried it. You would have to cook a pudding with the cream and sugar, then add the cocoa butter into it. I don't know what temperatures you must use to prevent it from separation, it will be finicky.
Considering all of the above, plus you reluctance to use milk powder, I would leave the whole "make my own white chocolate" idea. Purchase a proper white chocolate bar, add 14 to 18 ml of heavy cream, and presto. You have your ganache.
